There is a growing need for clean energy in the world, and one of the cleanest and most abundant sources is wind energy. Horizontal axis wind turbines have traditionally been utilized to harness wind energy, but they suffer from drawbacks. For example, current horizontal axis wind turbines have low efficiency due to large gears and transmissions. Parasitic power is consumed by the constant reorientation of the blades into the wind stream. Horizontal axis wind turbines are unable to harness turbulent winds, and are susceptible to damage in high winds.
Vertical axis wind turbines are characterized by the axis of blade rotation being at a right angle to the wind direction. Vertical axis wind turbines have been in use for many decades, and include drag-based designs that move by being pushed by the wind, and lift-based designs which move from lift that is developed by the vanes. The principal advantage of a vertical axis wind turbine is that it does not require alignment with the direction from which the wind is blowing, thus saving the cost of additional motors and controllers. However, several common problems have stymied efforts to bring vertical axis wind turbines into broad commercial acceptance.
One noted problem with vertical axis wind turbines is that they suffer inefficiencies due to drag during part of the rotation, which is a consequence of the vane shapes and gearing. One proposed solution was a wind turbine design that utilized flat fans mounted pivotally on a support structure to catch wind and cause the support structure to rotate. As the fans orbited the vertical axis, they pivoted between a downwind orientation, presenting a broad area that catches the wind, and an upwind orientation in which a narrower profile passes before the wind in order to create less drag. One noted drawback to that design was that the flat fans were not very aerodynamic in design, and thus operation was rough and slow, with the fans being pulled out of position by centrifugal force. The fans provided drive only intermittently during a somewhat small portion of each rotation.
Another solution was a wind turbine design that employed flat vanes on a rotating frame. The two-phase vanes were balanced on the vertical axis so that they pivoted about 170 degrees between a high-drag position downwind and a low-drag position upwind. The wind turbine exhibited drag rotation over 180 degrees of each revolution, but vane interference of the upwind vane over the downwind vane in its wind shadow reduced overall effectiveness. Thus, the effective transference of force occurred over less than 180 degrees.
Still other solutions were attempted utilizing a lift-based design. For example, one solution disclosed a lift-based vertical axis wind turbine that included vertically arranged vanes mounted pivotally on a rotating base. As the vanes caught the wind and moved the support, they orbited the vertical axis. A wind-vane-controlled pitch adjustment continually oriented the airfoils relative to the wind direction. The device detected wind direction by means of a vane, and positioned the controlling pitch flange accordingly. One drawback to this approach was that the positioning of the airfoils was only effective in the directly windward and directly leeward positions, using crosswind lift force in both cases.
Another solution involving a lift-based vertical axis wind turbine was to use “free flying” airfoils, wherein the airfoils were self-positioning according to the local dynamic conditions to which they were subjected, thereby creating a condition of equilibrium in order to make the engine more efficient. More specifically, the literature disclosed a vertical axis wind engine with a rotor mounted on a base for rotation about a vertical axis. One or more airfoil(s) were mounted on the rotor so that it was free to pivot between preset first and second limits of pivotal movement (e.g., set by stop mechanisms). That arrangement enabled the airfoil to align according to the wind as it orbited the vertical axis, thereby achieving better conversion of wind energy to useable rotational energy by combining lift and drag characteristics at low speeds and shifting to lift-only characteristics at rotor speeds approaching or exceeding local wind speed. Wind forces and armature-constraining action established airfoil positions. The airfoils rotated freely through an arc of approximately 90 degrees, bounded by stop mechanisms. The span of travel was from a radial line along the mounting arm (radially aligned relative to the vertical axis) to a perpendicular position (tangentially aligned relative to the vertical axis). The design allowed for each airfoil to set its own instantaneous angle and to adjust to conditions of relative wind, wind shift, and so forth occurring outside and within the wind engine, without external adjustments or mechanisms, wind vanes, centrifugal governors, or other controlling devices. Individual airfoils adjusted to local conditions based on changes of rotor speed, turbulence, true relative wind, and other factors affecting each of them independently. A drawback to this design, however, was that the efficiency was limited because the airfoils rotated through only about a 90 degree arc (out of a possible 360 degrees) and were constrained by stops.
A further drawback to the various vertical axis wind turbines is similar to those inefficiencies found in the horizontal models, namely that there is a relatively large amount of weight carried by the bearings that support vertically rotating components. In addition to the loss of energy resulting from the friction between the relative components, this leads to the need to replace bearings on a regular basis.